I am in the taper weeks of training for my second ultramarathon attempt (the first was the North Face Endurance Challenge 50k in DC last year), which means I'm spending far less time running and far more time thinking.

People talk of "taper madness," and it's a real thing: runners (and other event athletes, I suppose, but I don't do other events) claim that in the weeks before a race, every little muscle twinge feels like the portent of a major injury, every day feels long and stretchy with elasticized time, every short run leaves your legs feeling like they are pulsing with energy, ready to go further, go faster, go harder.

Taper madness is a real thing. But what it fails to account for is the opportunity inherent in tapering. This is the time to think about what we are doing, and why. Why would anyone want to run more than a marathon? (Why would anyone want to run a marathon, for that?)

I had a dentist appointment a few weeks ago, and the doctor saw me a full 45 minutes late. I had rushed out of a meeting to get to the appointment on time, and forgotten my phone, and realized I left my wallet at home, so had 45 minutes of time to sit and steam a bit. But actually, I didn't steam. Because even though I'd forgotten essentials like phone and wallet, I'd managed to grab my current book, Dubliners. So I had 45 uninterrupted, unreachable moments to sit and read Joyce's short stories. What a gift.

When the doctor did finally come in, she apologized briefly for "running a bit behind," and struck up the requisite small talk when one has a book in her lap: Oh, what are you reading?

Ever wondered why you find yourself able to sprint the last hundred meters of a 5k race, when you spent most of the third mile feeling like you couldn’t possibly take one more step? Or why crowd support makes you run better? Or why people tend to collapse after they cross the finish line of a marathon, rather than before?

Alex Hutchinson, columnist for Outside and Runner’s World, tackles these questions and more in his new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Hutchinson organizes his research into the limits of human performance into three buckets: Mind & Muscle (with chapters on how the brain interacts with our muscle capacity), Limits (pain, muscles, oxygen, heat, thirst, and fuel), and Limit Breakers (the science of training the brain to go beyond what we think we can do).